Metal clad molybdenum article



Dec. 4, 1962 F. A. MALAGARI. JR 3,066,393 METAL GLAD MOLYBDENUM ARTICLE Filed Feb. 17, 1958 INVENTOR.

Frank AvMologarhdr.

Patented Dec. 4-, 1962 3,066,593 METAL (JLAD MQLYBDENUM AR'HQLE Frank A. Malagari, lira, Leechhurg, Pat, assignor to Allegheny lLudlum Steel Corporation, Breckenridge, Pa, a corporation of Pennsylvania Filed Feb. 17, 1953, S83. No. 715,639 '7 Qlaims. (Cl. 29-193) This invention relates to clad molybdenum and more particularly to molybdenum objects clad with oxidation resistant nickel or iron base metals.

Material requirements of military and industrial advancernent, particularly in the metals field, are demanding metals and alloys with improved high temperature properties. An example of such demand is the necessity to acquire materials with high mechanical properties at temperatures of up to 2OOG F. and greater for uses such as turbine blades for jet aircraft. Molybdenum has been found to be an ideal metal for such applications from a mechanical standpoint. iowever, molybdenum is a highly reactive metal, the oxide of which is volatile at temperatures of about 900 F. and higher. The oxide of molybdenum forms rapidly at these temperatures. For these reasons, molybdenum may not be employed for such high temperature applications except where provision is made to protect the metal against oxidation.

it has long been sought to devise a means to adequately coat molybdenum and alleys of molybdenum with a material that will withstand oxidation at the desired high temperature while permitting the molybdenum to provide a the required mechanical properties. in the past, attempts have been made to employ corrosion resistant stainless steels or iron base alloys and corrosion resistant nickel base alloys, such as lnconel, as cladding for molybdenum. In the prior processing cladding is generally applied by abrasively cleaning the surfaces of the contacting metals and then forming a box-like structure by placing the cladding on either side of the molybdenum and welding side bars around the sides. The box-like structure is then evacuated of air and the entire assembly is hot rolled. Molybdenum and molybdenum alloy objects clad in this manner have generally been unsatisfactory due to separation of the clad from the molybdenum during rolling. The reason for such separation is generally conceded to be due to a brittle phase that forms at the interface of the two dissimilar metals where the molybdenum diffuses into the iron or nickel. iron and molybdenum when alloyed with one another in certain concentration ranges, that are certain to appear in the diffusion Zone of the clad metal objects, form the brittle phase. Nickel and molybdenum form a similar though not quite so severe a condition. Stresses caused by uneven thermal expansion and contraction of the clad metal in relation to the metal base or core when the bimetallic object is repeatedly heated and cooled during rolling cause the clad metal to shear from the core in the vicinity of the brittle phase. Occasionally such separation is so complete the cladding is completely separated from the core before rolling to the desired shape is completed.

Various attempts have been made to overcome the difficulties encountered in bonding iron and nickel base alloys to molybdenum. All, however, have been relatively unsuccessful. Among such attempts has been to interpose a barrier metal between the core and the cladding that will prevent molybdenum from diffusing or alloying with the clad metal. For example, chromium is often employed as such a barrier metal. However, metals such as chromium have proved to be unsatisfactory for such use due to the fact that molybdenum diffuses through the barrier and into the clad metal forming the brittle phase with the clad metal.

It has now been found that by employing tungsten or tungsten base alloys as the barrier metal between molybdenum or molybdenum alloy and with iron and nickel base cladding a highly satisfactory bond is obtained both between the molybdenum and tungsten and the clad metal and tungsten. Clad molybdenum sheets made by the method of the present invention show excellent bonding without stress cracking or shearing due to thermal expansion or contraction.

It is therefore an object of the present invention to provide a clad molybdenum body that possesses a superior bond between the cladding and the core.

Another object of the present invention is to provide a barrier metal that will form a good bond to molybdenum and iron and nickel base alloys so that iron and nickel base alloys may be successfully employed as clad metal for molybdenum.

A still further object of the present invention is to supply a layer of tungsten between molybdenum or molybdenum base objects and iron and nickel base clad cladding.

Other objects and advantageous features will be obvious from the following description when taken in conjunction with the accompanying drawing, the single FlGURE of which is a view in perspective with parts broken away showing an assembled pack embodying features of this invention.

In general, the present invention relates to a clad molybdenum structure whereby oxidation resistant stainess steels and nickel base alloys are bonded to molybdenum, and tungsten or tungsten alloys are employed as a barrier metal interposed between the clad metals and the molybdenum or molybdenum alloy core.

In the practice of the present invention iron and nickel base alloys that possess high oxidation resistance at elevated temperatures are bonded to molybdenum and molybdenum base alloys that exhibit highly desirable mechanical properties at elevated temperatures by employing tungsten or tungsten base alloy as a barrier metal between the core and clad. The resulting compact or clad metallic object possesses both the high temperature properties of. molybdenum and molybdenum alloys while at the same time possessing the high oxidation resistance of the clad stainless steel and nickel base alloys. Thus, there is created an ideal article or object for high temperature use employing the best properties of both materials, such object exhibiting a continuous and sound bond between the core and barrier metal and the barrier metal and the clad metal.

The preferred process of producing such a clad metallic object is best described by referring to the single figure of the drawing. As illustrated, a molybdenum alloy core it is interposed between two sheets 13 of tungsten barrier metal. The core 11 and barrier metal 13 are, in turn, interposed between two layers 15 of iron or nickel base clad metal. The compact is enclosed with side bars 17 which may be composed of the same material as cladding 15 or which may be of some metal of adequate stren th and which may be Welded to clad metal 15 and wr 1th possesses properties that will enable one to hot roll it to the desired gauge. Side bars 17 frequently are sheared from the object after rolling. Side bar 17 is provided with a circular perforation as shown at 21 through which a tube 19 projects outwardly to act as a passageway to evacuate the compact. After welding side bars 17 to cladding l5 and welding tube 19 into the opening of side bar 17 the assembly is checked for leaks, a positive pressure of argon being generally maintained inside the assembly until it is evacuated. When thus assembled and tested for leaks, the assembly is evacuated at a temperature somewhere between room temperature and the hot rolling temperature. The degree of evacuation may be dependent upon the quality of material desired but usually a vacuum of about at least microns of mercury is employed. Thereafter the evacuated tube 19 is forged shut and may be severed several inches from the side bar.

The evacuated assembly is then hot rolled to the desired gauge and is generally stress relieved after hot rolling. The hot rolling consists of one pass per reheat and initial reductions are generally heavy. The hot rolling temperatures for molybdenum and molybdenum alloys containing small amounts of alloys to raise the temperature of recrystallization are generally in the range between about 2000 F. to about 2250 F.

The molybdenum core may be of a commercially pure molybdenum or may be a molybdenum base alloy depend ing upon the exact high temperature properties desired. For example, it is common practice to employ small additions to molybdenum of such metals as titanium, zirconium, or columbium which tend to increase the temperature of recrystallization. Such alloying additions are generally employed because molybdenum possesses superior high temperature properties in its uncrystallized state. Thus, it is desirable to increase the temperature of recrystallization in molybdenum for high temperature use.

The cladding metal is preferably stainless steel, either possessing an austenitic structure such as those stainless steels containing a high nickel content or the so-called ferritic and martensitic stainless steels in which the only essential alloying ingredient is chromium. The preferred stainless steels are those that contain a relatively high chromium content in that the high chromium content steels exhibit far superior oxidation resistance at elevated temperatures than those possessing a relatively low chromium content. Thus it may be said that the preferred iron base clad metals are those that contain from about to chromium. The desired nickel base alloys may be any of the high temperature oxidation resistant alloys and may also contain alloying ingredients which provide additional high temperature oxidation resistance or enhance the mechanical properties such as chromium, molybdenum and iron. Excellent results have been obtained by employing stainless steels such as AISI types 310 and 446 and nickel base alloys such as Inco 702. lnco 702 is the designation given by the International Nickel Company of New York, NY. to an alloy of the following approximate analysis: carbon about .02%, manganese about 0.10%, iron about 0.35%, nickel about 80.00%, chromium about 15.00%, aluminum about 3.00% and titanium about 0.50%.

The thickness of the barrier metal employed is not critical in that molybdenum will not diffuse through the tungsten layer if there is a continuous layer. Excellent results have been obtained while employing the method set forth above when using a barrier metal with thicknesses as small as .005 inch. Instead of using thin sheets of tungsten or tungsten alloy interposed between the molybdenum and cladding metal, it may be advantageous to form the layer or barrier by electroplating tungsten onto the molybdenum core or onto the iron and nickel base clad metal. Continuous adherent coatings thus may be obtained as thin as .001 inch. The maximum thickness of the layer of barrier material is of no consequence except for practical consideration. For example, it would seldom be advantageous to employ a barrier metal that is as thick or thicker than the clad metal itself.

Microexamination of clad metallic objects prepared in the manner described above shows a sound metallic bond to exist between the molybdenum or molybdenum base alloy and the tungsten or tungsten base alloy barrier metal. These two metals are completely miscible in one another thus forming solid solutions upon diffusion without a brittle phase occurring. The dense tungsten layer prevents the molybdenum from migrating through the barrier and into the clad metal. The bond between the clad metal and the tungsten layer is likewise found to be sound in that up to and including 30% tungsten is soluble in nickel, and tungsten also forms a limited solid solution with iron. There has been detected between the tungsten barrier and the iron or nickel clad metal a white phase which as yet is undetermined as to its nature. However, this phase has proved to be sufficiently ductile in that it does not detract from the bond between the metals. It has also been found that an additional barrier metal such as chromium or nickel may be employed be tween the tungsten barrier metal and the clad metal if such a pratice is deemed to be desirable.

The following specific examples are given to illustrate the production of the clad metallic object of the present invention.

Experimental packs such as described above were made up. The core metal consisted of a molybdenum alloy that contains .5% titanium. These assemblies consisted of 1 /2 inch by 1 /2 inch compacts. One inch by one inch pieces of molybdenum core and tungsten barrier metal were employed. The cladding was cut to the overall size of the pack. Prior to assembly all compacts were cleaned of foreign material by hand rubbing with coarse and fine grades of emery paper. After welding, the assemblies consisted of .210 inch to .225 inch molybdenum core more completely surrounded by 4 inch type 304 stainless side bar and covered with .062 inch cladding. The cladding metal in the different compacts were types 310 and 446 stainless steel and lnco 702. The tungsten was placed between the cladding and the core. Several of the packs had in addition to the tungsten a .010 inch sheet of pure nickel placed between the cladding metal and the tungsten barrier. For evacuation purposes a /8 inch hole was drilled through one side bar and a A inch 0.1). stainless steel tube was welded thereto, thus completing the socalled bar-type assembly. 1 he assembly, before welding and rolling, was similar to that shown by the attached drawing. All packs Were evacuated at 2250 F. for a period of time sumcient to reach an equilibrium pressure. The evacuation tube was hand forged approximately 3 inches from the side bar and the pack was air cooled.

Rolling was conducted at 2200 F. to 2250 F. and the packs were stress relieved for 30 minutes at 2000" F. to 2250" F. The best results were obtained by cross rolling the sample. All samples were rolled to from .050 inch in gauge to .060 inch gauge. The bond interfaces of all the specimens were examined metallographically. All samples exhibited good bonding between molybdenumtungsten and tungsten-clad metal interfaces. Metallographic examination thus has shown a good bond is obtained when the method of the present invention is employed.

I claim:

1. A clad metal article comprising, a core of a metal consisting essentially of molybdenum, a barrier layer of a metal consisting essentially of tungsten, and a clad layer of at least one metal selected from the group of a first metal that consists essentially of iron and a second metal that consists essentially of nickel.

2. A clad metal article comprising, a molybdenum core, a tungsten barrier metal, and a stainless steel clad layer.

3. A clad metal article comprising, a core of a metal consisting essentially of molybdenum, a barrier layer of a metal consisting essentially of tungsten, and a stainless steel clad layer.

4. A clad metal article comprising, a core of a metal consisting essentially of molybdenum, an inner barrier layer of a metal consisting essentially of tungsten, an outer barrier layer of chromium, and a stainless steel clad layer.

5. A clad metal article comprising, a molybdenum core, a tungsten barrier metal, and a nickel clad layer.

6. A clad metal article comprising, a core of a metal consisting essentially of molybdenum, an inner barrier layer of a metal consisting; essentially of tungsten, an outer barrier layer of chromium, and a clad layer of a metal consisting essentially of nickel. 5

7. A clad metal article comprising, a core of a metal consisting essentially of molybdenum, a barrier layer of a metal consisting essentially of tungsten and being at least .001 thick, and a clad layer of an oxidation resistant metal selected from the group of a first metal that 10 consists essentially of iron and a second metal that consists essentially of nickel.

References Cited in the file of this patent UNITED STATES PATENTS Henderson Oct. 4, Van Gessel Apr. 10, Ruben Nov. 27, Eitel June 18, Hensel July 1, Van Geel Feb. 2, Volterra May 1, Kernpe Sept. 25, Pompa Nov. 19,

Bartlett Oct. 7, 

1. A CLAD METAL ARTICLE COMPRISING, A CORE OF A METAL CONSISTING ESSENTIALLY OF MOLYBDENUM, A BARRIER LAYER OF 